1. Field of the Invention
The present invention relates to nanoparticles of transition metals, and more particularly to convenient chemical syntheses of stable, monodisperse elemental (such as hexagonal close-packed (hcp), face-centered Cubic (fcc), and a novel cubic phase of cobalt, alloy (Co/Ni, NiFe, Co/Fe/Ni,) (where relative concentrations of the elements can vary continuously) and intermetallic (Co.sub.3 Pt, CoPt, CoPt.sub.3, Fe.sub.3 Pt, FePt, FePt.sub.3 etc. which are distinct compounds with definite stoichiometries), and overcoated magnetic nanocrystals (e.g., particles consisting of a concentric shell of material of different chemical composition produced by a serial process) preferably having sizes substantially within a range of about 1 to about 20 nm.
2. Description of the Related Art
Magnetic properties of fine particles are different from those of bulk samples due to a "finite size" effects.
Specifically, with the finite size effect, as the particle size is reduced from micrometer to nanometer scale, the coercive forces increase and reach a maximum at the size where the particles become single-domain.
Potential applications of small magnetic particles include not only ultra-high density recording, nanoscale electronics, and permanent magnets, but also their use as novel catalysts, in biomolecule labeling agents and as drug carriers. An important goal related to each of these potential applications is to make monodisperse magnetic domains with high durability and corrosion resistance.
A variety of physical and chemical synthetic routes have been attempted to produce stable, monodisperse zero-valent magnetic nanocrystals. These include sputtering, metal evaporation, grinding, metal salt reduction, and neutral organometallic precursor decomposition.
Conventionally, controlling the particle size of nanostructured metal clusters has been limited only to late transition metals, such as Au, Ag, Pd and Pt particles. The early transition metal particles prepared according to conventional methods are either in aggregated powder form or are very air-sensitive, and tend to agglomerate irreversibly. This is problematic because the air sensitivity generates safety concerns when large quantities of the materials are present, and results in degradation over time due to oxidation unless expensive air-free handling procedures are employed during processing and the final product is hermetically sealed. The irreversible agglomeration of the particles makes separation processes which could narrow the size distribution impossible, and prevents the ready formation of smooth thin films essential in magnetic recording applications. The agglomeration reduces the chemically-active surface for catalysis, and seriously limits the soluability essential for biological tagging, separation and drug delivery applications.
Thus, precise control of particle dimensions and making monodisperse nanocrystals remain important goals in technological applications of nanomaterials. Ferromagnetic uniaxial Cobalt-based nanomaterials (e.g., many of these materials are tetragonal crystal structures which like the hcp structure is uniaxial) (e.g., such as CoPt inter-mettalics, and Co/Ta/Cr alloy) have been used in high density recording media, while fcc cobalt-based nanoparticles or Ni/Fe alloy particles are magnetically soft materials with low anisotropy which is advantageous in the development of read heads and in magnetic shielding applications. It is noted that the terms "hexagonal close-packed (hcp)" and "face-centered cubic (fcc)" refer to the specific internal crystal structure of the particles and is important determining the anisotropy of the magnetic properties. Additionally, these materials are anticipated to display interesting, giant (e.g., very large) magnetoresistive properties when organized in extended arrays, and thus are candidates, for example, for magnetoresistive read head sensors.
Moreover, previously, the reproducible chemical synthesis of magnetic transition metal nanocrystals uniform to better than about 5% in diameter has been difficult or impossible. The inability to control nanocrystal size to better than 5% has in turn frustrated any efforts to prepare 2- and 3-dimensional ordered assemblies of these uniform transition metal and metal alloy nanocrystals. Traditional methods for the preparation of metal nanocrystals include physical methods such as mechanical grinding, metal vapor condensation, laser ablation, electric spark erosion, and chemical methods included solution phase reduction of metal salts, thermal decomposition of metal carbonyl precursors, and electrochemical plating.
When any of these physical or chemical processes is performed directly in the presence of a suitable stabilizing agent and a carrier fluid or the metal particle deposited from the vapor phase into a carrier fluid containing a suitable stabilizer, a magnetic colloid (e.g., ferrofluid) may result. All of the above-mentioned techniques have been practiced for many years and have been unable to refine the level of control needed for the production of stable magnetic colloids of transition metals and metal alloys to the levels demonstrated by the present inventors.
Several factors have limited the efficacy of the existing techniques. First, the technical difficulty involved in the isolation/purification of the magnetic colloids is high, and in fact only in the last decade have the tolerances for the performance of materials and devices based on magnetic materials and devices narrowed to make uniformity in size to better than 5% a distinct advantage. Secondly, the tremendous growth in magnetic technology in medical and biotechnology industries has opened many new applications.
Thus, the conventional techniques have been unable to exercise the required control in the production of stable magnetic colloids of transition metals and metal alloys. The poor chemical stability of the conventional metal particle has limited the reliability of systems in which they are incorporated and has prompted wide-scale use of the metal oxide nanoparticles in many applications despite the weaker magnetic properties inherent in the metal oxide particles.